Tiny circuit big boost for electronics

Images of graphene integrated circuits are shown here. On top is an optical image of a completed graphene mixer including contact pads. On the bottom is a scanning electron image of a top-gated, dual-channel graphene transistor used in the mixer integrated circuit.

Wireless communications took a small leap forward today with the announcement that researchers have created a functional integrated circuit smaller than a grain of salt.

The circuit is a broadband frequency mixer, which is "one of the most fundamental and important circuits in essentially all wireless communication devices and equipment," Yu-Ming Lin, an IBM researcher who led the effort, told me today.

Mixers, for example, convert low-frequency audio signals into high-frequency signals that can be transmitted wirelessly. The new circuit is made of graphene, the Nobel Prize worthy crystalline material made with a single layer of carbon atoms.

The research community has been abuzz over graphene for the past few years because it is the strongest crystalline material yet known, can be stretched like rubber and is an excellent conductor of heat and electricity.

It is being eyed for a range of technologies such as lighter and cheaper body armor, touchscreen displays and chemical sensors.

Last month, we reported on the use of the material in an optical modulator, which switches light on and off and thus has the potential to serve as a blazingly fast broadband data pipe. Today, Lin and colleagues report in Science the integration of a graphene transistor onto a silicon carbide wafer.

"This is a circuit component that has a real function, a practical use in a real application, for example the cellphone," Lin said. "The significance is, because of the integration, the entire circuit can be very small; in this case less than 1 millimeter squared."

Compared to silicon, the graphene transistor could be less expensive, use less energy and free up room inside portable electronics such as smart phones, where space is tight, the researchers note.

Until now, researchers have been unable to integrate graphene transistors with other components on a single chip primarily due to poor adhesion of graphene with metals and oxides and the lack of a fabrication scheme to yield reproducible devices and circuits.

Lin's team overcame these hurdles by developing wafer-scale fabrication procedures that maintain the quality of graphene and, at the same time, allow for its integration to other components. This is how IBM describes what they did:

In this demonstration, graphene is synthesized by thermal annealing of SiC wafers to form uniform graphene layers on the surface of SiC. The fabrication of graphene circuits involves four layers of metal and two layers of oxide to form top-gated graphene transistor, on-chip inductors and interconnects.

The circuit operates as a broadband frequency mixer, which produces output signals with mixed frequencies (sum and difference) of the input signals.

Mixers are fundamental components of many electronic communication systems.

Frequency mixing up to 10 GHz and excellent thermal stability up to 125°C has been demonstrated with the graphene integrated circuit.

Lin told me that this mixer circuit serves as a stepping stone to "a wide variety of more sophisticated and complicated circuits." For example, they could be integrated with medical imaging devices used for detecting cancer cells.

Going forward, the team will continue to improve the performance of the mixer and work on a more complex layout — more transistors on the chip, for example, enabling increased functionality.

The integrated circuit was invented by Jack Kilby at Texas Instruments in 1958, a feat that earned him the Nobel Prize in Physics in 2000. That circuit had just one transistor but paved the way to the highly complex circuits used in electronics today, Lin noted.

"With that analogy, this is really one of the first stepping stones to a new function based on graphene," he said.